1. Field of the Invention
This invention relates to cermet filters used to filter combustion products from internal combustion engines. More particularly, this invention relates to a porous, intermetallic-ceramic composite filter that is self-cleaning and can improve emissions from internal combustion engines.
2. State of the Art
Emissions from vehicles, such as heavy-duty diesel engine vehicles, contribute greatly to pollution problems of the United States of America (USA). Heavy-duty vehicle emissions produce ozone, particulate materials (PM), nitrogen oxides (NOx), sulfur oxides (SOx), and volatile organic compounds (VOCs). These emissions can cause adverse health effects such as premature mortality, aggravation of respiratory and cardiovascular disease, chronic bronchitis, changes to lung tissues and structures, and altered respiratory defense mechanisms, among other things. Further, ozone is known to cause crop and forestry losses and PM causes damage to materials and soiling. NOx and PM also significantly contribute to unsightly smog and substantial visibility impairment in many parts of the USA.
Emissions from diesel engine heavy-duty trucks significantly contribute to these problems throughout the country. By 2007, heavy-duty vehicles will account for 29 percent of NOx and 14 percent of PM emissions in the USA. These proportions are even higher in some urban areas. For example in Albuquerque, N. Mex., heavy-duty vehicles contribute 37 percent of NOx and 20 percent of PM emissions.
The United States Environmental Protection Agency (EPA) is proposing a PM emissions standard for new heavy-duty engines of 0.01 grams per brake-horsepower-per hour (g/bhp-hr) to take full effect in 2007. The EPA is also proposing more stringent standards for NOx and VOC emissions to be phased in between 2007 and 2010. In order to meet these rigorous new standards, new and improved filters are needed.
Engine and catalyst manufacturers have experimented with many catalytic converters and with a wide variety of regenerative catalytic traps. Precious metal catalytic traps are somewhat effective in oxidizing gaseous hydrocarbons and CO as well as the particulate soluble organic fraction (SOF). However, precious metal catalysts are very expensive. Base metal catalytic traps promote soot oxidation but have little effect on NOx, COx, or SOx.
Intermetallic-ceramic catalyst supports have been produced in the past. For example, U.S. Pat. No. 5,951,791 to Bell et al. discloses using nickel aluminide to coat the inside of an alumina fiber preform. U.S. Pat. No. 5,774,779 to Tuchinskiy and U.S. Pat. No. 4,990,181 to Pierotti et al. disclose using nickel aluminide as a catalyst support. U.S. Pat. No. 4,992,233 to Swaroop et al. discloses using iron aluminide alloys in exhaust filter applications. Also, U.S. Pat. No. 5,496,655 to Lessing discloses using a porous NiAl or Ni3Al with a ceramic filler to catalyze steam reforming of hydrocarbons to power fuel cells.
Porous ceramic filters made from carbide and oxide materials are well known in the art. However, conventional ceramic filters used with heavy-duty diesel engines have a significant lifetime problem. These ceramic filters have a short lifetime due to severe carbon particulate plugging and structural failure due to high vehicle motion stresses and extreme thermal stresses. Typically, the filters are plugged after 500 hours of service, which makes them a financial and operational liability for companies. Further, the filters often require complicated heating and control systems.